completion: let 'for-each-ref' filter remote branches for 'checkout' DWIMery

The code listing unique remote branches for 'git checkout's tracking
DWIMery outputs only remote branches that match the current word to be
completed, but the filtering is done in a shell loop iterating over
all remote refs.

Let 'git for-each-ref' do the filtering, as it can do so much more
efficiently and we can remove that shell loop entirely.

This speeds up refs completion for 'git checkout' considerably when
there are a lot of non-matching remote refs to be filtered out.
Uniquely completing a branch in a repository with 100k remote
branches, all packed, best of five:

  On Linux, before:

    $ time __git_complete_refs --cur=maste --track

    real    0m1.993s
    user    0m1.740s
    sys     0m0.304s

  After:

    real    0m0.266s
    user    0m0.248s
    sys     0m0.012s

  On Windows, before:

    real    0m6.187s
    user    0m3.358s
    sys     0m2.121s

  After:

    real    0m0.750s
    user    0m0.015s
    sys     0m0.090s

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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